Whatever I do in Pose Mode of my Rig, Tail of the Rig goes crazy
There is a Parenting loop.
What is happening is the green bone at the very top of your tail is copying the rotation of a bone that is parented to it (I cannot find it for some reason) this bone is called tail control. When your tail bone rotates, it rotates the control bone, which rotates the parent bone, which rotates the control bone, and so on.
What fixes the problem is simply remove the bone constraint from the green bone. If you still want to keep the copy rotation, just add a bone into your scene, press alt+p to remove parent, and use the new bone you created.